Gas turbine case and reinforcement strut for same

ABSTRACT

A case assembly for a gas turbine engine comprising annular case components each having a central axis. Radial struts each have a radial axis and intersect the annular case components. A stress dissipation mass projecting from a continuous surface of at least one of the struts at the intersection with a corresponding annular case component, the stress absorption mass being on either side of a plane passing through the radial axis of the strut and the central axis of the corresponding annular case component. A method for dissipating thermal and mechanical stresses on a strut in a case assembly for a gas turbine engine is also provided.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a divisional application of U.S. patent application Ser. No. 14/135,651 filed on Dec. 20, 2013, incorporated herewith by reference.

TECHNICAL FIELD

The present application relates to a gas turbine engine and to a case therefore.

BACKGROUND OF THE ART

Turbofan engines typically have a case assembly with a fan case, an intermediate case, a compressor case, a gas generator case, a turbine case and a turbine exhaust case about a centreline. The hot section of the engine, including the gas generator case, the turbine case and turbine exhaust case, are typically made of steel or nickel alloys. The cooler sections such as the intermediate case and the compressor case may be made of lighter materials such as aluminium or magnesium. However, steel is conventionally used for the fan case because of its strength.

In the intermediate case, there is a compressor air passage, and a bypass air passage, defined by an annular splitter. The splitter extends forward of radial struts integrated between the intermediate case and the fan case. The intermediate case with the splitter and the struts is generally integrally cast and then machined. The parts are of uniform material thickness resulting in above limit stresses in certain locations.

Low cycle fatigue is a persistent problem which can reduce the useful life of various structural components due to cycling between idling and operating conditions and thus subjected to stresses generated by thermal expansion effects and mechanical loads. Due to the high loads experienced on intermediate case designs, low cycle fatigue locations are especially noted on all struts. Increasing the mass to compensate increases the thermal stresses, while decreasing mass increases the g loading and thrust contribution.

Improvement in case design is desired.

SUMMARY

In one aspect, the present disclosure provides a case assembly for a gas turbine engine comprising: annular case components each having a central axis; radial struts each having a radial axis, the radial struts intersecting the annular case components; and a stress dissipation mass projecting from a continuous surface of at least one of the struts at the intersection with a corresponding annular case component, the stress absorption mass being on either side of a plane passing through the radial axis of the strut and the central axis of the corresponding annular case component.

In another aspect, the present disclosure provides a method for dissipating thermal and mechanical stresses on a strut in a case assembly for a gas turbine engine comprising: determining a location of substantial thermal and mechanical stresses, determining a volume of mass required to dissipate the thermal and mechanical stresses at the location, and forming a discrete mass on the strut protruding from a continuous surface of the strut, at the determined location on either side of the strut.

In yet another aspect, the present disclosure provides A gas turbine engine comprising: a case assembly of an annular case components each having a central axis; radial struts in the case assembly, each having a radial axis, the radial struts intersecting the annular case components; and a stress dissipation mass projecting from a continuous surface of at least one of the struts at the intersection with a corresponding annular case component, the stress dissipation mass being on either side of a plane passing through the radial axis of the strut and the central axis of the corresponding annular case component.

Further details of these and other aspects of the present invention will be apparent from the detailed description and figures included below.

DESCRIPTION OF THE DRAWINGS

Reference is now made to the accompanying figures depicting embodiments of the present invention, in which:

FIG. 1 is a schematic cross-sectional view of a turbofan gas turbine engine;

FIG. 2 is a fragmentary perspective view of a strut and splitter assembly with a detail in accordance with an embodiment; and

FIG. 3 is a fragmentary, enlarged perspective view of the detail shown in FIG. 2;

FIG. 4 is a schematic view showing a bulge size in the presence of a weldline for the detail of FIG. 2; and

FIG. 5 is a schematic view showing a bulge size in the absence of a weldline for the detail of FIG. 2.

DETAILED DESCRIPTION

Referring to FIG. 1, a turbofan gas turbine engine which is an exemplary application of the described subject matter includes an engine outer case 10, a core case 11, a low pressure spool assembly (not indicated) which includes a fan assembly 12, a low pressure compressor assembly 13 and a low pressure turbine assembly 14 connected by a shaft 15, and a high pressure spool assembly (not indicated) which includes a high pressure compressor assembly 16 and a high pressure turbine assembly 17 connected by a turbine shaft 18. The core case 13 surrounds the low and high pressure spool assemblies to define a main fluid path (not numbered) therethrough. The high and low pressure spool assemblies co-axially define a rotational engine axis X of the engine 10.

It should be noted that the terms “radial”, “axial” and “circumferential” used throughout this specification and appended claims, unless otherwise specified, are with respect to the engine axis X.

As shown concurrently in FIGS. 1 and 2, an intermediate case 22 is illustrated having an inner hub 24 and an outer ring 26. The inner hub 24 may be mounted onto the turbine shaft 18 to support the turbine shaft 18 when it rotates. The intermediate case 22 may be immediately downstream of the fan case surrounding fan 12 as shown in FIG. 1. A plurality of struts 28 extend from the inner hub 24 to the outer ring 26.

Splitter ring 30 separates the bypass air flow from the flow entering the compressor section (FIG. 1), with the flow entering the compressor section being radially inward of the bypass air flow. The splitter ring 30 is supported by the struts 28, and may have a gaspath baffle 31. A support ring 32 may also be connected to the struts 28 (e.g., welded) between the inner hub 24 and the splitter ring 30, and may be used to support a bleed-off valve, among other possibilities.

Referring now to FIGS. 2 and 3, a stress dissipating mass 36 (i.e., stress distribution mass) is positioned at the joint 34 between one of the struts 28 and the splitter ring 30, which joint 34 typically comprises a fillet. The stress dissipating mass 36 is formed by a pair of bulges 36 a, 36 b (a.k.a., lobes), placed symmetrically, one on either side of a plane extending in the radial axis of the strut 28 and the longitudinal axis of the inner hub 24 (i.e., the engine axis X). The bulges 36 a, 36 b mirror geometries, although they may not be mirror images of one another as well. According to an embodiment, the stress dissipating mass 36 may be machined from the stock forming the strut 28, or may have other constructions as well. As shown in FIG. 3, the strut 28 may comprise a flange-like portion 38 to contact the splitter ring 30, with the bulges 36 a and 36 b at the junction between the main radial portion of the strut 28 and the flange-like portion 38. The flange-like portion 38 of the strut 28 may be welded to the splitter ring 30 along weld lines 40, among other possibilities. The distance between the bulges 36 a, 36 b and the weld line 40 is established to avoid the weld bead being close to the bulge radius.

The bulges geometry may be proportional to the strut leading edge fillet radius, to spread the load in front of the strut 28. The minimum width (in the tangential direction, also referred to as length) may be equivalent to the strut leading edge fillet radius. The lobe width should not exceed 2 times the strut fillet radius. Larger lobes will add weight to the part without any further stress reduction.

In the embodiment in which there is no welded joint in front of the strut 28 (e.g., weld line 40), the bulges 36 a, 36 b may be longer. A suitable maximum length may be one time the strut leading edge fillet radius.

In an embodiment, the bulges 36 a, 36 b are not in the gas path, as they are underneath the gaspath baffle 31 to avoid disturbing the gas flow. Hence, the height of the bulges 36 a, 36 b may be smaller than a height of the baffle 31. Stated differently, the bulges 36 a, 36 b are used to spread the load in front of the strut 28. The load and thus the stress was concentrated in the strut leading edge area. The stress dissipating mass 36 redistributes the load without adding extra thickness all over the splitter ring 30 and thus without adding excessive weight.

Referring to FIG. 4, one of the bulges 36 a is shown being about 0.150 in away from the weld line 40 to avoid having double stress concentration (the distance being given as an example). The bulge 36 a has a height h that may be about 3 times the ring thickness to have significant stiffness change to transfer the stress away from the leading edge of the strut 28.

Referring to FIG. 5, an exemplary embodiment is shown in which there is no weld line at the junction between strut 28 and splitter ring 30. In such a case, the length L of bulge 36 a may be increased, for instance up to about 3 times the strut leading edge fillet radius RL. Also, the radius RG of the bulge 36 a may be increased to reduce the stress concentration

The discretely selected, increased mass from the bulges 36 a, 36 b dissipates the thermal and mechanical stresses at the joint of the strut 28 and the splitter ring 30, without adding significant weight to the assembly. The location of the stress dissipating mass 36 at the junction between the strut 28 and the splitter ring 30 may stiffen the overall carcass from bending. Moreover, the junction between the strut 28 and the splitter ring 30 may be a critical location in terms of fatigue, whereby the stress dissipating mass 36 strengthens the junction. It is contemplated that the stress dissipating mass 36 be applied in other case sections, for instance the exhaust case 20. The stress dissipation mass 36 may be defined as a protuberance on the surface of the strut 28, which would otherwise be a generally continuous and arcuate junction between two generally planar surface. The stress dissipating mass 36 is radially inward oriented relative to the splitter ring 30. Due to its location and relatively low profile, the stress dissipating mass 36 does not have a significant on gas flow.

The above description is meant to be exemplary only, and one skilled in the art will recognize that changes may be made to the embodiments described without departing from the scope of the invention disclosed. Other modifications which fall within the scope of the present invention will be apparent to those skilled in the art, in light of a review of this disclosure, and such modifications are intended to fall within the appended claims. 

1. A method for dissipating thermal and mechanical stresses on a strut in a case assembly for a gas turbine engine comprising: determining a location of substantial thermal and mechanical stresses, determining a volume of mass required to dissipate the thermal and mechanical stresses at the location, and forming a discrete mass on the strut protruding from a continuous surface of the strut, at the determined location on either side of the strut.
 2. The method as defined in claim 1, wherein forming the discrete mass comprises machining the discrete mass from material native to strut.
 3. The method as defined in claim 1, wherein forming the discrete mass comprises forming the discrete mass at an intersection of the strut and an annular radial component. 